Good Evening Signalnorth
This depends on how the locomotive was created
Most creators tend towards using the 'anim' or 'anim2' smoke-modes. These (mostly anim2, being designed specifically for steam locomotives) give the appropriate 'chuffs' when taking off, and for anim2 then turns to a nice 'full' smoke stream when over a certain wheel RPM or speed (can't remember which off-hand). However, these are 'constant' effects, being that they will always be on when the locomotive is moving.
Trainz does include a smoke-mode called 'stack' which turns the smoke on/off depending on the regulator and cut-off positions. However, this tends to only give smoke when working really hard, and then nothing once you start to back off a bit. For the most part, it doesn't really give the effect most creators want these days.
Some creators, however, have taken to scripting their smoke effects to tie the 'anim2' effects with the regulator and cut-off. This is something I've been implementing on my last two payware Australian steam locomotive releases, the latest pack having 4 different effects to represent the look of the steam/smoke based on the regulator and cut-off. Similar systems have been implemented by other creators as well recently, both freeware and payware but mostly for US content.
Regards
